Conventionally, a wet-type friction engaging apparatus has been used in such as an automatic transmission for a motor vehicle or a transmission for an industrial or construction machinery etc. as a apparatus to transmit torque or to turn off its transmission.
As shown in FIG. 4, a friction disk 30 and a mating plate 40 are arranged alternately in a drum D of a wet-type friction engaging apparatus and the drum D houses a piston P, which is thrust by operating oil A, pressing the friction disk 30 and the mating plate 40 to be in press contact with each other, and a spring receiver SR, which receives a spring S driving the piston P away from the friction disk 30 and the mating plate 40.
The wet-type friction engaging apparatus 10A transmits torque in its engaging state when the piston P thrusts the friction disk 30 and the mating plate 40, and turns off its transmission of torque in its releasing state when the piston P is driven away from the friction disk 30 and the mating plate 40.
When the wet-type friction engaging apparatus is in its engaging state, the friction disk 30 and the mating plate 40 generate heat to high temperature since they are rubbed against and in press contact with each other. Therefore, the friction disk 30 and the mating plate 40 are cooled down by circulated cooling/lubricating oil B.
A projection which is provided on a opening end 22 of an inner surface of a hub 20 prevents the lubricating oil B from leaking from its end regardless of rotational speed, and thus, all lubricating oil B, regardless of rotational speed, drains through an oil bore 24, be circulated.
However, since such a wet-type friction engaging apparatus 10A needs to be supplied a large amount of lubricating oil B in order to heighten the cooling effect of the friction disk 30 and the mating plate 40, and it causes a problem such that the driving force of an oil pump (not shown) has to be increased.
In addition, if the amount of supply of lubricating oil B is increased in order to heighten the cooling effect, lubricating oil B is regularly churned by the friction disk 30 or the mating plate 40 in the releasing state of the wet-type friction engaging apparatus 10A, and it causes a problem that fuel efficiency would worsen due to so-called “friction loss”.
Moreover, since the cooling of the friction disk 30 and the mating plate 40 by the lubricating oil B is required during high-speed rotation, the lubricating oil is often overly supplied during low-speed rotation, which is a usual state of use. Therefore, in a usual state of use, a drag torque is generated due to the viscosity of the lubricating oil B and this causes drag loss and lowering of efficiency.
On the other hand, there is a lubrication and cooling structure of a wet-type clutch (by way of example, refer to Unexamined Japanese Patent Publication No. JP 10-9287A), which may sufficiently cool the engaging section of a wet-type friction engaging apparatus only with a small amount of lubricatinq oil.
This comprises a lubricating oil storage and a lubricating oil supply path, characterized in supplying lubricating oil to the lubricating oil storage through the lubricating oil supply path, and draining it through an opening of the lubricating oil storage.
In this connection, although the title of the invention is “lubrication structure,” in JP 10-9287A, the object of the lubricating oil is not only to reduce abrasion of a friction section, but also to release heat, in other words, cool the friction section, and thus it should be referred to as a “lubrication and cooling structure” to be more precise.
It is described therein that according to the invention, sufficient cooling/lubricating oil is supplied to a friction surface; heat is sufficiently transmitted to the lubricating oil; the lubricating oil in the lubricating oil storage moves certainly from an outer diameter side of the friction surface to the opening of the lubricating oil storage to be drained; therefore, heat is not stored in lubricating oil; and as a result, the friction surface may be sufficiently cooled down with only a relatively small amount of lubricating oil.
It is also described therein that when the lubricating oil storage has a lubricating oil drain port, which opens and drains lubricating oil in the lubricating oil storage when friction engaging elements are not engaged, the drag torque of a clutch may be lowered at the time when friction engaging elements are not engaged, and thus, the loss of motive power can be reduced and fuel efficiency can be improved.
However, the lubrication and cooling structure of JP 10-9287A has a difficulty of miniaturizing a wet-type friction engagement apparatus since its structure is complicated and a lot of structural elements are required so as to realize its function.
In view of the above problems, it is an object of the present invention to provide a lubrication and cooling structure of a wet-type friction engaging apparatus, of which structure is relatively simple, and which may reduce drag torque at the time of a usual state of use.